1. Field of the Invention
The present invention relates to a signal equalizer for adjusting the intensity of an initial signal in each of a plurality of frequency bands, and particularly relates to a sound signal equalizer such as a bass and treble audio tone control circuit or a graphic equalizer.
2. Description of the Related Art
In a sound apparatus, a graphic equalizer and bass and treble audio tone control circuit are used to perform boosting that increases the level of a prescribed frequency band or cutting that reduces the level of a prescribed frequency band.
FIG. 1 is a circuit diagram showing the schematic configuration of a conventional bass and treble audio tone control circuit. The circuit has a treble band block 2 and a bass band block 4 that are connected in series between an input terminal IN and an output terminal OUT, and the sound signal inputted to the input terminal IN sequentially passes through the treble band block 2 and the bass band block 4 and is outputted from the output terminal OUT. The circuit is composed of principal parts integrally configured on a semiconductor substrate as an integrated circuit, and external parts that are connected to external terminals 6, 8, and 10 of the integrated circuit.
The treble band block 2 boosts or cuts the high-frequency component of the sound signal. A capacitor C1 is connected to the external terminal 6 of the treble band block 2. Switches SW1 and SW3 are switched on when boosting is carried out. The treble band block 2 thereby constitutes a non-inverting amplifier having a differential effect and amplifies the high-frequency component of an inputted sound signal. Switches SW2 and SW4 are switched on when cutting is performed. The treble band block 2 thereby constitutes a low-pass filter (LPF) and attenuates the high-frequency component of an inputted sound signal.
On the other hand, the bass band block 4 boosts or cuts the low-frequency component of the sound signal. Capacitors C2 and C3 and a resistor R1 are connected to the external terminals 8 and 10 of the bass band block 4. Switches SW1 and SW3 are switched on when boosting is carried out. The bass band block 4 thereby constitutes a non-inverting amplifier having an integrating effect and amplifies the low-frequency component of an inputted sound signal. Switches SW2 and SW4 are switched on when cutting is performed. The bass band block 4 thereby constitutes a high-pass filter (HPF) and attenuates the low-frequency component of an inputted sound signal.
The treble band block 2 and bass band block 4 are each provided with a number of resistors that are connected in series inside an integrated circuit. The series resistors 12 and 14 are divided into various ratios in accordance with which of the switches disposed in a plurality of locations is switched on, and the boosting or cutting gain can thereby be adjusted.
Also, the frequency band that is to be boosted or cut by the treble band block 2 and bass band block 4 is determined by the resistance value and capacitance value of the external components and the resistance value that is set in accordance with the manner in which the series resistors 12 and 14 are divided. In this case, the resistance value of the series resistor 12 and the capacitor C1 must be set to relatively large values in order to set the frequency band that the treble band block 2 is to process in a high-frequency region of, e.g., about 10 kHz or higher, in a sound signal. In the same manner, the resistance value of the series resistor 14 and the capacitors C2 and C3 must be set to relatively large values when the frequency band that the bass band block 4 is to process is set in a low-frequency region of, e.g., about 100 kHz or lower, in a sound signal.
In an integrated circuit, the number of pins and other external terminals is limited, and the number of external components may need to be reduced due to the package size or other restrictions. A reduction in the number of external components would lead to expectations of fewer assembly steps, lower costs, and other advantages. From such a viewpoint, it is possible to consider housing the capacitors C1 and C3 inside the integrated circuit.
However, the capacitors C1 to C3 must be set to a relatively large value in the manner described above. There is therefore a problem in that a large surface area is required on the semiconductor substrate and the chip size is increased when the capacitors are accommodated in the integrated circuit. In this case, the capacitances of C1 to C3 can be reduced by increasing the resistance values of the series resistors 12 and 14 without changing the boosting or cutting frequency characteristics, but doing so results in a problem in that the surface area required by the series resistors 12 and 14 is increased. An equivalent resistance composed of switched capacitors can be used to implement a considerable resistance value in a relatively small surface area. However, there is a problem in that the large number of resistors connected in a series must be individually composed of switched capacitors in the series resistors 12 and 14 composed of resistors as described above, causing the circuit to be made more complicated and the size to increase. Due to these problems, C1 to C3 are not easily accommodated in an integrated circuit.    [Patent Document 1] Japanese Laid-open Patent Application No. H05-090926.